The invention relates generally to the field of medical imaging, and more specifically to the field of imaging dynamic, internal tissue, such as cardiac tissue, by computed tomography.
Non-invasive imaging broadly encompasses techniques for generating images of the internal structures or regions of a person or object that are otherwise inaccessible for visual inspection. For example, non-invasive imaging techniques are commonly used in the industrial field for inspecting the internal structures of parts and in the security field for inspecting the contents of packages, clothing, and so forth. One of the best known uses of non-invasive imaging, however, is in the medical arts where these techniques are used to generate images of organs and/or bones inside a patient that would otherwise not be visible.
One class of non-invasive imaging techniques that may be used in these various fields is based on the differential transmission of X-rays through a patient or object. In the medical context, a simple X-ray imaging technique may involve generating X-rays using an X-ray tube or other source and directing the X-rays through an imaging volume in which the part of the patient to be imaged is located. As the X-rays pass through the patient, the X-rays are attenuated based on the composition of the tissue they pass through. The attenuated X-rays then impact a detector that converts the X-rays into signals that can be processed to generate an image of the part of the patient through which the X-rays passed based on the attenuation of the X-rays.
One such X-ray imaging technique is known as computed tomography (CT). CT imaging systems measure the attenuation of X-ray beams passed through the object from numerous angles. Based upon these measurements, a computer is able to process and reconstruct images of the portions of the object responsible for the radiation attenuation. As will be appreciated by those skilled in the art, these images are computed by processing the angularly displaced projection data to generate cross-sectional and/or three-dimensional reconstructions of the imaged object or region. Such reconstructions may be displayed on a monitor and/or may be printed or reproduced on film.
CT imaging techniques, however, may present certain challenges when imaging dynamic internal tissues, such as the heart. For example, in cardiac imaging, the motion of the heart causes inconsistencies in the projection data, which, after reconstruction, may result in various motion-related image artifacts such as blurring, streaking, or discontinuities. In particular, artifacts may occur during cardiac imaging when projections that are not acquired at the same point in the heart cycle, i.e., the same phase, are used to reconstruct the image or images that comprise the volume rendering.
To avoid the image artifacts associated with cardiac motion, therefore, it is desirable to reconstruct projection data acquired at the same phase into the desired images. This may be done by selective acquisition of the projection data (prospective gating) or by selecting and reconstructing only projection data acquired at the same cardiac phase (retrospective gating). Such gating techniques may utilize a simultaneously acquired electrocardiogram (ECG) signal that is used to acquire select projection data, either prospectively or retrospectively, at a common phase of cardiac motion. In this example, prospective gating refers to modulating data acquisition, such as the X-ray tube output and projection data acquisition, in response to real-time measurement and analysis of the ECG signal, i.e., acquiring only the projections of interest at a specified phase of cardiac motion for reconstruction. Similarly, in this example, retrospective gating refers to using the temporal correspondence between the ECG signal and the acquired projection data, to select only that projection data corresponding to a particular phase of the ECG signal for reconstruction.
However, an ECG signal is a measure of the depolarization and repolarization of the cardiac muscle tissue. While the electrical cardiac events measured by an ECG are generally indicative of cardiac muscle contraction and motion, this electrical activity is still only an indirect indicator of cardiac motion. Because of the indirect nature of this relationship, artifacts may still be present in the images reconstructed using gating techniques that rely upon ECG signals. It is desirable, therefore, to devise methods and apparatus to estimate cardiac motion using acquired CT projection data, without utilizing an ECG device. In addition, examining a patient without disposing electrodes on the patient and connecting the electrodes to an ECG device may advantageously result in increasing patient throughput, which is particularly beneficial in an emergency setting.